Live from the Hubble Space Telescope

UPDATE # 2

Many schools are engaged in preparing birthday cards for Dr. Clyde Tombaugh,
who turns 90 in a few days. Dr. Tombaugh discovered Pluto in 1930. February
9 is the deadline for submitting your creations for a presentation to
Dr. Tombaugh. For details about this activity, see the first updates-hst
message. Several wonderful student-created cards are already online on
the Web in the area called Kids' Corner.
Please have a look if you are Web capable.

Also, to correct something from the activity description: We provided
an incorrect address to the commentary entitled "Clyde Tombaugh's Blinking
Persistence". The correct address is http://www.jpl.nasa.gov/pluto/vol1a.htm
(that is vol one-a)

Please accept our apologies for any inconvenience this mistake caused.

January 23, 1996
The votes are in and the students have spoken. After tallying up all the
counts and discussing the options, it was decided that two of the "Live
From HST" (LHST) orbits will be used to image Neptune. Now my job was
to decide exactly how to use the time in those two orbits.

Using the Hubble Space Telescope (HST) is sort of like using a very
complicated camera. In fact, the instrument we are going to use for LHST
is called the "Wide-Field Planetary Camera." This camera is actually four
cameras together, but we are going to use just the part called the "Planetary
Camera," because it has the best resolution (it will show us the best
details of the clouds on Neptune).

Unlike the cameras that most people have, which use color film, the
Hubble Space Telescope takes pictures with electronic cameras, which are
black and white. To get colors with Hubble, we take pictures through different
filters. Filters are pieces of glass that are specially treated to let
just certain colors though the glass. So you can get a "blue" picture
through a "blue" filter or a "red" picture through a "red" filter, depending
on which filter you choose. (By the way, we can use a computer to COMBINE
pictures taken through different filters to make a "true-color" picture
to show what Neptune would look like if a person could actually look THROUGH
the Hubble Space telescope - but remember, no people physically look though
Hubble - it is in outer space, and we use computers to "look" through
it).

Neptune looks very different depending on which color you look at it.
(That's not unusual: most things do - you can check this yourself with
plastic wrap of different colors.) On Neptune, the Great Dark Spots are
mostly visible in the blue, but the bright spots are most visible in the
red. You have to be very careful about exactly which filters you pick.
I picked five different filters that range from blue to very red. That
way, we should be able to see bright spots, dark spots, and anything else
interesting.

Once we decide what filters (or colors) we will be using, we have to
decide the exposure time. This means, how long will we need to keep the
camera's shutter open to make a picture. Most regular cameras figure that
out automatically using some kind of light sensor. With Hubble (and actually
with any fancy camera) you must figure this out yourself. For fancy cameras,
this is because the photographer can then make the picture look exactly
like she wants. In the case of Hubble, it is both that, and also that
the targets are very faint, so standard sensors wouldn't work. Anyway,
we are in luck, because I have used Hubble in the past to look at Neptune
already, so I know what the right exposure times should be, and they range
from 14 seconds in one filter up to 400 seconds (almost 7 minutes) for
the red filters.

We have talked about what filters we will use, and how long the exposures
will be. But we still need to think about the fact that we have two orbits.
My plan is to use the same set of filters for each of the two orbits,
and to time the two orbits so that we see as much of Neptune as possible.
If you were going to make a map of the whole Earth from outer space, you'd
take one picture, then wait 12 hours and take another picture, since you
know that the rotation period of the Earth is 24 hours. Why can't we do
the same kind of thing for Neptune? The problem is that on Neptune all
we see are the tops of the clouds, and the winds that move these clouds
have different speeds at different parts of the planet. So there is not
just "one" rotation period. What should we use, then? We will use the
rotation period from the part of the planet where we see the most clouds.
Since the rotation period there is about 16 hours, we will space our two
orbits 8 hours apart.

When exactly should we take the data? Which two PARTICULAR Hubble orbits
do we want? Well, we cannot say for sure exactly which we will get, because
there are a lot of observations that need to be made with Hubble Space
Telescope. What we can do is request to have our data taken within a certain
time (within a certain week, for example). Then the scientists at the
Space Telescope Science Institute compare our request with the requests
from everyone else who wants to use Hubble around that time, and they
use a computer program to put all of them together in the most efficient
way possible.

On March 14, we will be having a live TV broadcast about the LHST data,
so to make this program as exciting as possible, I will request that the
first orbit be taken 9 hours before the live broadcast, and that the second
orbit be taken just before the broadcast, so that the data are sent down
from the spacecraft DURING the broadcast! But remember, we may not get
this exactly - it really depends on what else the Space Telescope has
to look at on that day. In any case, if all goes well our pictures will
be taken sometime in the day or so before the live broadcast. They will
be very fresh and brand-new pictures of Neptune! I hope you are excited
about this as I am!

Heidi Hammel

NOTE: Heidi Hammel has prepared a document which includes the technical
details of the planning discussed above. To review those details of our
Neptune observations, take a look on the Web here.

(If written, this URL should be on one line, without any space or breaks)